basically whenever I join a server on css, my game and vent go really quiet, because windows 7 automatically turns down the volume levels in the volume mixer to really low for some reason.

So I have to tab out, turn them back up again and it's fine. But I have to do this each map change / every time I play the game and it gets very annoying.

Does anyone know a solution to this / had this problem ever before at all?

It's a feature. Go into sound settings and look at the "Communications" tab, and set it to "Do nothing".

This should help

Thanks, hopefully this will fix the problem